As soon as you arrive, go to immigration, ask for permission to open bank account to apply for retirement Visa, take that permission letter to Bangkok Bank, no other bank will do it for you
After you have your visa on your passport, then you can open with any bank.
As I understand, NO agent can do it, anyone dare, they will arrest them.
Go to immigration, tell them you are applying for a visa ( pick one that you will qualify for and require bank account balance) Ask them for a letter of permission,
If you are inside the country, there are other visa you can apply, go to immigration, ask for a letter of permission to open account, base on your application visa type that requires account balance. Then go to Bangkok Bank.
Well, if you want, go to Huahin, I will give you a contact to help you do it, she is an agent, she is very good and has contact in Immigration. And most importantly, she is very honest.